Pressure sensing apparatus

Abstract

An etched metal membrane has its rim laser-welded or electron-beam-welded to a ring made of the same kind of metal within which a central electrode is held by a metal-to-glass seal. The welding is done in vacuum to avoid the necessity of later evacuation. The pressure sensor capacitance thus formed is connected through an impedance converter to an oscillator circuit which provides an output frequency that varies with the sensed pressure. The inner surface of the central electrode is flush with the surface of the rim to which the membrane is welded to reduce variations in capacitance of the sensor, and any residual variations are compensated by a resistance balance in the evaluation circuit. The insensitivity of the system to temperature and its service life make it suitable for sensing the intake air pressure of an internal combustion engine.

We claim: 
     
       1. Apparatus for sensing pressure, capable of responding to changes of intake air pressure of an internal combustion engine and comprising: a metallic membrane (4) formed of an etched metal platelet to provide an attachment rim for said membrane slightly and uniformly in front of the peripheral portion of said membrane;   a metal ring (1), to a flat surface of which said rim of said membrane is joined by metal-to-metal fusion in an airtight, pressure-resistant and electrically conductive manner;   an electrode (3) located centrally in said metal ring bonded to said metal ring by a single pressure-tight fused glass seal, insulated from said metal ring and said membrane, and having a surface coplanar with said flat surface of said metal ring, and   connections to said metal ring and to said electrode for using said membrane and said electrode as a pressure-sensitive capacitor in an electrical evaluation circuit.   
     
     
       2. Apparatus as defined in claim 1 in which said membrane (4) and said metal ring (1) are made of the same metallic material. 
     
     
       3. Apparatus for sensing pressure, capable of responding to changes of intake air pressure of an internal combustion engine and comprising: a metallic membrane (4) formed of an etched metal platelet to provide an attachment rim for said membrane slightly and uniformly in front of the peripheral portion of said membrane;   a metal ring (1) to a flat surface to which said rim of said membrane is joined in an airtight, pressure-resistant and electrically conductive manner;   an electrode (3) located centrally in said metal ring bonded to said metal ring by a pressure-tight fused glass seal, insulated from said metal ring and said membrane, and having a surface coplanar with said flat surface of said metal ring, the spacing between said membrane and said glass seal being greater than the spacing between said membrane and said electrode, whereby the volume of the cavity between said membrane and said electrode is increased, and   connections to said metal ring and to said electrode for using said membrane and said electrode as a pressure-sensitive capacitor in an electrical evaluation circuit.   
     
     
       4. Apparatus for sensing pressure, capable of responding to changes of intake air pressure of an internal combustion engine and comprising: a metallic membrane (4) formed of an etched metal platelet to provide an attachment rim for said membrane slightly and uniformly in front of the peripheral portion of said membrane;   a metal ring (1) to a flat surface of which said rim of said membrane is joined in an airtight, pressure-resistant and electrically conductive manner;   an electrode (3) located centrally in said metal ring bonded to said metal ring by a pressure-tight fused glass seal, insulated from said metal ring and said membrane, and having a surface coplanar with said flat surface of said metal ring;   an evaluation circuit including an RC oscillator circuit and an impedance converter for reducing the input resistance and input capacitance of said RC oscillator circuit, said impedance converter having two field effect transistors (9,10) as active elements thereof, and   connections to said metal ring and to said electrode for using said membrane and said electrode as a pressure-sensitive capacitor in said electrical evaluation circuit.   
     
     
       5. Apparatus as defined in claim 4 in which said evaluation circuit includes a network for compensation of circuit and input capacitances, said network comprising: an operational amplifier (12) having an inverting and a non-inverting input;   a capacitor (13) connected with one of said inputs of said operational amplifier, and   two feedback resistances (14, 15) both connected to the output of said operational amplifier and respectively connected to said two inputs of said operational amplifier.   
     
     
       6. Apparatus as defined in claim 5 in which one of said resistances is variable, whereby variation produced in manufacture in the value of the range of the capacitance between said membrane (4) and said centrally located electrode (3) may be compensated for.